1. Field of the Invention
The present invention generally relates to symbol synchronizing circuits for synchronizing symbols of received signals, and particularly relates to a symbol synchronizing circuit for synchronizing symbols of received OFDM signals.
2. Description of the Related Art
In the OFDM (orthogonal frequency division multiplex) transmission, carrier waves that are arranged at constant frequency intervals and orthogonal to each other are subjected to digital modulation at symbol frequency, thereby achieving the allocation of data. Modulation and demodulation in the OFDM are achieved by performing the IFFT with respect to a plurality of symbol data items at the transmission end, and by performing the FFT with respect to the received signal at the reception end. To this end, the reception apparatus needs to detect the window timing at which the FFT is applied to the received signal. This necessitates highly accurate detection of symbol timing with respect to the OFDM symbols.
FIG. 1 is an illustrative drawing showing the format of a preamble portion of an OFDM signal according to IEEE802.1a. As shown in FIG. 1, the beginning of a frame is provided with 10 short training symbols t1-t10 that are identical to each other and arranged at a predetermined interval (a 16-data-sample length, which is referred to as a symbol sample interval), followed by a guard interval GI2 of a 32-data-sample length and long training symbols T1 and T2 that are identical to each other and 64-data-sample long. In order to detect the symbol timing of OFDM symbols, it suffices to detect the timing of an end of t10, i.e., the timing of an end of the short-training-symbol portion.
FIG. 2 is a block diagram showing the construction of a related-art apparatus for detecting symbol timing. The construction shown in FIG. 2 is disclosed in Non-patent Document 1, and includes a symbol timing detecting unit 10, an A/D converter 11, and a signal detecting unit 12. The symbol timing detecting unit 10 includes a matched filter 13, an autocorrelation electric power computing unit 14, a synchronous summation unit 15, a moving average unit 16, and a symbol-start detecting unit 17. The A/D converter 11 performs A/D conversion on the received OFDM signal for provision to the signal detecting unit 12, the matched filter 13, and the autocorrelation electric power computing unit 14. The matched filter 13 performs a matched-filter process on the signal supplied from the A/D converter 11 based on a signal waveform supplied from a known-signal memory circuit, which stores therein a signal waveform identical to that of a short training symbol. As a result, the output of the matched filter 13 ends up having a peak at the boundary of each short training symbol. This output signal is subjected to synchronous summation performed at symbol-sample intervals by the synchronous summation unit 15, and is also subjected, on a separate path, to the generation of moving averages by the moving average unit 16. The outcome of synchronous summation and the obtained moving averages are supplied to the symbol-start detecting unit 17, respectively. The autocorrelation electric power computing unit 14 computes the autocorrelation electric power of a signal supplied from the A/D converter 11, and supplies the computed autocorrelation electric power to the symbol-start detecting unit 17.
The symbol-start detecting unit 17 detects a provisional start point based on the outcome of synchronous summation, and makes a final determination as to the position of an end of the short-training-symbol portion by checking the levels of autocorrelation electric powers and moving averages. This achieves highly accurate detection of symbol timing with respect to OFDM symbols.
[Patent Document 1]
Japanese Patent Application Publication No. 11-168446
[None-Patent Document 1]
Tomoya Tandai, Kazumi Sato, and Minoru Namekata, “A Study of frame synchronization method for IEEE802.11a system,” Proceedings of the 2002 IEICE General Conference, Mar. 7, 2002, Communication 1, p. 704
The related-art construction shown in FIG. 2 has a drawback in that it is sensitive to noise because fixed thresholds are used for the checking of levels of synchronous summations, autocorrelation electric powers, and moving averages. The synchronous detection of symbols by use of fixed thresholds suffers a significant drop in accuracy, especially in a multi-path configuration where multi-path delay waves are superimposed on a received signal, thereby creating large fluctuation in received electric power. Since the multi-path configuration is almost always required in the field of radio communication, some countermeasures need to be devised to prevent receivers from suffering a significant drop in their performance.
As a measure to cope with the fluctuation of a received signal level in the multi-path environment, there is a technology (Patent Document 1) that changes a threshold for use in checking when detecting a null period in a set of synchronous symbols based on received-signal electric power. Such a change in the threshold is made in response to the moving average of received-signal electric power. This technology, however, is only directed to the threshold-based checking of signal electric power, and is silent about synchronous detection based on other signals. That is, only the threshold of signal electric power used at the time of null-period detection is adjusted based on the signal electric power itself, and there is no teaching or suggestion of threshold-based checking using other signals. This technology thus cannot be applied to the OFDM format as shown in FIG. 1.
Accordingly, there is a need for an OFDM symbol synchronizing circuit which achieves highly accurate threshold-based checking of a plurality of signals in the synchronizing of OFDM symbols.